Mixing processes



P 16, 1969 c. BAILEY ETAL 3,467,362

MIXING PROCESSES Filed Oct. 4, 1967 FIG. I. couw BAILEY MICHAEL E.N.EVANS Inventors ZS Z l7/9r Attorneys United States Patent Office 3,467,362 Patented Sept. 16, 1969 us. or. 2's9 14 6 Claims ABSTRACT OF THE DISCLOSURE A free non-interfering dispersion of fibrous material, particularly fragile material such'sas ceramic whiskers which are damaged by conventional equipment, in viscous liquids is formed by introducing the fibres and a relatively much larger quantity of viscous liquid into a cylindrical drum free of internal projections so asto occupy at most about half the volume of the drum, and rotating the drum about its longitudinal axis with said axis horizontal so that fibres and liquid rotate with a side of the drum for part of each revolution only and then drop back whereby the shear forces created in the liquid disperse the fibres.

This invention relates to mixing processes. It is concerned with providing a process for forming a free noninterfering dispersion of fibres in a viscous including a visco-elastic liquid. Fibrous material with which the invention is particularly concerned includes asbestos fibres and whiskers such as silicon nitride and silicon carbide whiskers. It may also be used for"disper'sing such fibres as glass,.cotton and wool fibres. All these fibres are normally supplied in a randomly close-packed bulk.

When attempts are made to disperse fibrous material in liquids more viscous than water with conventional equipment, such as vessels having rotating stirrers or rotating vessels having vanes, knots of matted fibres tend to form around the stirrer and inadequate dispersion results. Furthermore, brittle fibres such as certain crystal whiskers are liable to be damaged by the mechanical action of this type of conventional equipment.

The present invention provides a method of forming a dispersion of fibres in a viscous'liquid, the dispersion being substantially free of knotted or interfering fibres i.e. substantially all the fibres are completely surrounded by liquid. It also minimises the possibility of damage to brittle fibres.

According to the present invention, a process for the dispersion of fibres in a viscous liquid"c omprises introducing the fibres and a relatively much larger quantity of the liquid into a cylindrical drum so as to occupy at most about half the volume of the drum, the drum being rotatable about a substantially horizontal axis and having inside surfaces which are substantially free from projections inwards from its cylindrical surface, the process involving rotating the drum about its axis at such a speed that the fibres and liquid rotate with a side of the drum for part of the revolution only and then drop back into the liquid, until the fibres are dispersed therein.

The fibres and liquid tend to rotate with the shell of the drum, and strong shear-forces are set up in the boundary layer of liquid adjacent to the shell of the drum. The more viscous the liquid, the thicker the boundary layer in which shear forces are set up. The shear forces act on any mass of fibres to tear it apart, fibre by fibre, and disperse the fibres. The preferred angular speed is that at which the fibres and liquid will leave the shell of the drum somewhat before, e.g. 3050 before, the highest point of travel of the shell. The preferred angular speed depends on several factors. It varies with the smoothness and shape of the shell. It varies inversely with the internal diameter of the shell and also inversely with the viscosity of the liquid jfibre mixture. Satisfactory dispersion of fibres in viscous and visco-elastic liquids is obtained with cylindrical drums which are circular in cross-section. Drums of polygonal cross-section may be suitable for use with liquids of medium viscosity and sticky fibres.

The length of time required to satisfactorily disperse fibres at the preferred speed varies with the flexibility and size of the fibres and the volume ratio of bulk of fibres to liquid and the volume ratio of liquid to drum. The time requiredis longer for longer flexible fibres. such as asbestos fibres which may be up to 5 cm. in length than for short thin fibres such as silicon nitride whiskers whose length is between about l000 microns.

The invention is particularly suitable for the preparation of a dispersion of fibres prior to aligning them by the process described in co-pending British patent application No. 39,954/64. A dispersion of fibres in a viscous liquid is passed through an orifice, whereby the fibres are aligned. For this and for other purposes, it is necessary to provide a dispersion which is substantially free of the dissolved air which would normally accumulate in the dispersion during the process of the present invention. According to features of the invention the liquid is deaerated before being placed in the drum, the fibres are placed in some of the liquid and dearated before being placed in the drum, the drum is partially evacuated of air after the liquid and fibres have been placed therein, and sealed prior to rotation in order to prevent gas from dissolving in the liquid during rotation.

The proce S.v may also be suitable for producing a dispersion of fibres in a carrier for use as a textured paint.

A particular process for forming a non-interfering dispersion of fibrous material in a liquid, in accordance with the invention, will now be described with reference to the accompanying drawing in which:

FIGURE 1 shows a perspective view of a cylindrical drum in which the non-interfering dispersion of fibrous material is formed, and

FIGURE 2 is a cross-sectional view through the cylindrical drum along the section line IIII.

3 lb. of ammonium alginate, 6 lb. of acetone (to swell the alginate), and 7 /2 lb. of deaerated water were mixed for 15 minutes in a vessel having a conventional rotating stirrer rotating at about 100 r.p.m. under vacuum conditions. The mixture was run into another vessel containing 62 /2 lb. of deaerated water and stirred conventionally at r.p.m. for /2 hour, under vacuum conditions.

Theniixtu re was poured into the stainless steel drum 1 having an internal length of 26 inches and an internal diameter of 20 inches through the scalable inlet 2.

1 lb. of crysotile asbestos fibres between about 1 and 3 cm. long were stirred conventionally in 20 lb. of deaerated water in a stand pot under vacuum conditions and the mixture was poured into the alginate solution already in the drum through scalable inlet 2.

Air was evacuated from the drum to a vacuum of 30 inches of water gauge via scalable inlet 2 and the drum was sealed and rotated for approximately 3 hours at 40 r.p.m., when a satisfactory dispersion of fibres was achieved.

The process was also carried out with 1 lb. silicon nitride Whiskers, between about 0.09 mm. and 0.3 mm. long, in place of asbestos fibres. The time required to achieve a satisfactory dispersion at 40 r.p.m. was 1 hr.

The ammonium alginate, acetone, water mixture is a visco-elastic liquid, whose viscosity varies from about 16.25 poises at a shear rate of 80 per second, through 10 poises at a shear rate of per second to 9.5 poises at a shear rate of 235 per second.

We claim:

1. A process for the dispersion of fibres in a viscous liquid which comprises introducing the fibres and a relatively much larger quantity of said viscous liquid into a cylindrical drum so as to occupy at most about half the volume of said drum, said drum having its longitudinal axis substantially horizontal and its inside surface substantially free from projections and being rotatable about said longitudinal axis; and rotating said drum about said longitudinal axis at a speed such that fibres and viscous liquid rotate with an inside surface of said drum for part only of each revolution and then drop back, whereby shear forces in said viscous liquid effect dispersion of said fibres.

2. A process according to claim 1 and in which said viscous liquid is deaerated before introduction into said cylindrical drum.

3. A process according to claim 1 and in which said cylindrical drum is partially evacuated of air after the introduction of fibres and viscous liquid and sealed prior to rotation.

4. A process according to claim 1 wherein said fibres are selected from the group consisting of asbestos fibres and silicon nitride whiskers.

5. A process according to claim 1 wherein said viscous liquid consists of a mixture of ammonium alginate, acetone and water.

6. A free non-interfering dispersion of fibres in a viscous liquid when produced by a process in accordance with claim 1.

References Cited UNITED STATES PATENTS 834,722 10/1906 Groesbeck 259-81 XR WALTER A. SCHEEL, Primary Examiner JOHN M. BELL, Assistant Examiner US. Cl. X.R. 

